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Influence of different surface conditioning protocols on microtensile bond strength of self-adhesive resin cements to dentin
Influence of different surface conditioning protocols on microtensile bond strength of self-adhesive resin cements to dentin Jatyr Pisani-Proença, DDS, MSc, PhD,a Maria Carolina Guilherme Erhardt, DDS, MSc, PhD,b Regina Amaral, DDS, MSc,c Luiz Felipe Valandro, DDS, MSc, PhD,d Marco Antonio Bottino, DDS, PhD,e and Ramón Del Castillo-Salmerón, DDS, PhDf School of Dentistry, University of Granada, Granada, Spain; School of Dentistry, Federal University of Rio Grande do Sul, Porto Alegre, Brazil; School of Dentistry, São Paulo State University at São Jose dos Campos, São Jose dos Campos, Brazil; School of Dentistry, Federal University of Santa Maria, Rio Grande do Sul, Brazil Statement of problem. According to manufacturers, bonding with self-adhesive resin cements can be achieved without any pretreatment steps such as etching, priming, or bonding. However, the benefit of saving time with these simplified luting systems may be realized at the expense of compromising the bonding capacity. Purpose. The purpose of this study was to assess whether different dentin conditioning protocols influence the bond performance of self-adhesive resin cements to dentin. Material and methods. Flat dentin surfaces from 48 human molars were divided into 4 groups (n=12): 1) control, no conditioning; 2) H3PO4, etching with 37% H3PO4 for 15 seconds; 3) SEBond, bonding with self-etching primer adhesive (Clearfil SE Bond); and 4) EDTA, etching with 0.1M EDTA for 60 seconds. The specimens from each dentin pre-treatment were bonded using the self-adhesive cements RelyX Unicem, Maxcem or Multilink Sprint (n=4). The resin-cement-dentin specimens were stored in water at 37°C for 7 days, and serially sectioned to produce beam specimens of 1.0 mm2 cross-sectional area. Microtensile bond strength (µTBS) testing was performed at 1.0 mm/min. Data (MPa) were analyzed by 2-way ANOVA and Tukey multiple comparisons test (α=.05). Fractured specimens were examined with a stereomicroscope (x40) and classified as adhesive, mixed, or cohesive. Additional bonded interfaces were evaluated under a scanning electron microscope (SEM). Results. Cement-dentin µTBS was affected by the dentin conditioning approach (P<.001). RelyX Unicem attained statistically similar bond strengths to all pre-treated dentin surfaces. H3PO4-etching prior to the application of Maxcem resulted in bond strength values that were significantly higher than the other groups. The lowest µTBS were attained when luting Multilink Sprint per manufacturers’ recommendations, while H3PO4-etching produced the highest values followed by Clearfil SE bonding and EDTA. SEM observations disclosed an enhanced potential of the self-adhesive cements to form a hybrid layer when applied following manufacturer’s instructions. Conclusions. When evaluated self-adhesive resin cements are used, selectively etching dentin with H3PO4 prior to luting results in the most effective bonding. (J Prosthet Dent 2011;105:227-235)
Research Fellow, Department of Stomatology, School of Dentistry, University of Granada. Associate Professor, Department of Conservative Dentistry, School of Dentistry, Federal University of Rio Grande do Sul. c Research Fellow, Department of Dental Materials and Prosthodontics, São Paulo State University at São Jose dos Campos. d Associate Professor, Department of Restorative Dentistry, School of Dentistry, Federal University of Santa Maria. e Chair and Professor, Department of Dental Materials and Prosthodontics, São Paulo State University at São Jose dos Campos. f Professor, Department of Prosthodontics, School of Dentistry, University of Granada. a
b
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Clinical Implications
Bond strength of evaluated self-adhesive resin cements is improved by pre-conditioning the dentin surface with H3PO4. Sufficient smear layer dissolution is necessary to enable chemical interaction and micro-mechanical inter-locking with the dentin surface. The presumed benefit of saving time with self-adhesive luting agents may only be realized at the expense of compromising bond strength. There is a growing interest in the use of self-adhesive resin cements. Manufacturers purport to combine an adhesive and a cement in a single application, eliminating the need for pre-treatment of both tooth and restoration.1 Bonding is thought to be primarily achieved by acidic monomers that demineralize and infiltrate the tooth substrate, resulting in mechanical retention.2 Secondary reactions have been suggested to provide chemical adhesion to hydroxyapatite, a feature currently only demonstrated in glass-ionomers.3 The simplification in handling achieved with these new agents is expected to make the luting procedure less technique- and operator-sensitive than when using 3-step systems.2,4 Thus, the concept of self-adhesive luting is attractive, because in theory, it is assumed that these cements should adhesively bond to smear layer-covered dentin without any pretreatment.1,2 Nevertheless, a limited etching potential and the ability to only superficially interact with dentin were observed for some self-adhesive resin cements,5-7 related to the high viscosity of the cement that would hinder deeper resin penetration.8 The diffusion of acidic resin monomers through the smear layer is relatively slow.9 Smear layers reinforced by impregnated resin may be too weak to provide strong, durable bonds.10-13 The mineralized components of the smear layer are efficient buffers,9,14 making the pH of the acidic monomers too high to demineralize the underlying dentin.15-17 Undeniably, more stable bonds may be formed if polymers can penetrate through den-
tal substrates less deeply, but more uniformly.18,19 The bonding mechanisms of different composite resins and adhesive monomers to tooth structures are the topic of several investigations.20-27 Therefore, it would be worthwhile to investigate if alternative bonding strategies,28-30 such as the use of different smear layer conditioning solutions (for example, strong or milder acids, self-etching adhesives) may be helpful in achieving better infiltration of dentin by self-adhesive cements, and as a consequence, improve the bond strength. The purpose of this study was to assess the impact of different conditioning agents, phosphoric acid, a mild self-etching adhesive system and EDTA, on the microtensile bond strength (µTBS) of 3 self-adhesive resin-based luting agents to dentin. The interaction of such bonding materials with dentin was also examined by scanning electron microscopy (SEM). The null hypotheses to be tested were: (1) dentin surface pre-treatments do not improve bond strengths of selfadhesive resin cements, and (2) higher µTBS does not depend on the luting agent used.
MATERIAL AND METHODS Forty-eight caries-free extracted human molars were used in this study. Teeth were stored in distilled water containing 0.5% chloramine T (Haldies Chemicals Pvt Ltd, Pune, India) at 4°C, and used within one month following extraction. The occlusal surfaces of the molars were ground flat with silicon carbide (SiC) metal-
The Journal of Prosthetic Dentistry
lographic papers (600-grit, Carborundum; Saint-Gobain Abrasivos Ltda, Guarulhos, SP, Brazil) under running water to obtain smear layer-covered dentin surfaces. Prior to the cementing procedures, a wax block (Cera Reus, S.A., Reus, Spain), 6 x 5 x 5 mm, was fabricated and subsequently impressed with vinyl polysiloxane (VPS) impression material (Elite HD; Zhermack, Badia Polesine, Italy), resulting in a matrix of VPS. A light-polymerized hybrid composite resin (Tetric EvoCeram, lot no. F38346; Ivoclar Vivadent, Schaan, Liechtenstein,) was incrementally inserted (2 mm), condensed into the templates and light polymerized for 40 seconds (Translux EC; Heraeus Kulzer GmbH, Hanau, Germany) until each template was completely filled. Light intensity output was monitored with a radiometer (Demetron Curing Radiometer; Kerr Corp, Orange, Calif ) throughout the bonding procedures to maintain at least 600 mW/ cm2. Polymerized composite resin blocks were polished with SiC papers up to 1200-grit under copious water and further cleaned ultrasonically (Vitasonic II; Vita Zanhfabrik, Bad Säckingen, Germany) for 5 minutes. The 48 flattened dentin surfaces were divided into 4 groups, according to the dentin pre-treatment to be applied (Table I): 1) no surface conditioning (control), the resin cements were applied directly to moist smear-layer covered dentin surfaces; 2) H3PO4, prior to luting, dentin was treated with 37% phosphoric acid (Total Etch; Ivoclar Vivadent) for 15 seconds, water rinsed for 10 seconds and left moist; 3) SEBond, a 2-step
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Table I. Manufacturers and composition of materials used in study Material (pH) RelyX
Manfacturer (Lot Number)
Composition
Application
3M ESPE,
Powder: glass powder, silica, calcium hydroxide,
Mix capsule for 15 s; apply on surface;
pigment, substituted pyrimidine, peroxy
lute composite resin block using pressure;
compound, initiator.
light polymerize for 40 s from each side.
Unicem
St. Paul, Minn
(pH 2.0)
(249122)
Liquid: methacrylated phosphoric ester, dimethacrylate, acetate, stabilizer, initiator. Maxcem
Kerr Corp,
(pH 2.3)
Orange, Calif (443517)
Unpolymerized methacrylate ester monomers,
Mix base and catalyst through automix
mineral fillers, activators, stabilizers and
dual-barrel syringe; apply on surface;
colorants.
lute composite resin block using pressure; light polymerize for 40 s from each side.
Multilink Sprint (pH 2.8)
Ivoclar Vivadent, Schaan, Liechtenstein (experimental lot)
Base and Catalyst: pastes of dimethacrylates
Auto-mix cement; apply on surface;
(24-26%), inorganic fillers, ytterbiumtrifluoride,
lute composite resin block using pressure;
initiators, stabilizers and pigments
light polymerize for 40 s from each side.
(5% methacrylated phosphoric acid ester). Total Etch
Ivoclar Vivadent,
(pH 0.6)
Schaan, Liechtenstein
35% phosphoric acid, fumared silica.
H3PO4 conditioning for 15 s. Rinse with water spray for 10 s, leaving tooth moist.
(JO8736) Clearfil SE
Kuraray Co,
Primer: 10-MDP; HEMA; hydrophilic
Apply Primer for 20 s; mild air stream.
Bond
Osaka, Japan
dimethacrylate; di-camphorquinone;
Apply Adhesive; gentle air stream.
N,N-diethanol-p-toludine, water.
Light polymerize for 10 s.
(00604A)
Adhesive: 10-MDP; Bis-GMA; HEMA; hydrophobic dimethacrylate; di-camphorquinone; N,N-diethanol-p-toluidine; silanated colloidal silica. 10-MDP=10-methacryloxydecyl dihydrogen phosphate; HEMA=2-hydroxyethyl methacrylate; Bis-GMA=bis-phenol A diglycidylmethacrylate.
self-etching primer adhesive (Clearfil SE Bond; Kuraray Co, LTD, Osaka, Japan) was bonded to the dentin prior to the luting procedures according to the manufacturer’s instructions; 4) EDTA, a 0.1 M EDTA (ethylenediaminetetraacetic acid) solution (pH 7) was placed onto dentin surfaces for 60 seconds; the surface was then rinsed with water spray for 10 seconds and left moist. After dentin surface treatments, teeth were re-assigned into 3 subgroups (n=4). Three self-adhesive luting agents were used: 1) RelyX Uni-
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cem, 2) Maxcem, and 3) Multilink Sprint. Materials were manipulated according to the manufacturer´s instructions. Composition, pH values and application protocol of the tested materials are listed in Table I. The pre-polymerized composite resin blocks were directly pressed on the cement using strong finger pressure and stabilized under a 750-g static load applied for 10 minutes.22 The excess cement was removed with a brush before light polymerizing with a halogen unit (Translux EC; Heraeus Kulzer GmbH) for four 40-second periods at
each side of the cemented composite resin blocks. Light intensity output was monitored with a radiometer (Demetron Curing Radiometer, Model 100; Kerr Corp) throughout the bonding procedures to be at least 600 mW/ cm2, and the light tip was positioned at 0.5 cm from the specimens during the polymerization process. After storage in distilled water at 37°C for 7 days, the resin-cementdentin specimens were longitudinally sectioned into serial slabs, and further sectioned into 1.0 ± 0.2 mm2 beams. Beams from each experimental group
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Volume 105 Issue 4 were individually attached to a microtensile bond strength apparatus with a cyanoacrylate adhesive (Super Bonder Gel; Henkel Loctite, Düsseldorf, Germany) and stressed to failure in tension using a universal testing machine (Model 5567; Instron Corp, Norwood, Mass) with a 10 Kgf load cell operated at a crosshead speed of 1.0 mm/min. The cross-sectional area at the site of failure was measured to the nearest 0.01 mm2 with a digital caliper (Starrett 727; L.S. Starrett, Athol, Mass). Fractured specimens were examined with a stereomicroscope (Olympus SZ-CTV; Olympus, Tokyo, Japan) at x40 magnification to determine the mode of failure. Failure modes were classified as adhesive, mixed, or cohesive. In the event of spontaneous debonding during handling, specimens were excluded from the statistical analysis.
Two additional resin-cementdentin specimens from each experimental group were prepared as previously described, and subsequently processed for morphological evaluation by scanning electron microscopy. Two mm-thick slabs containing the coupled interfaces were sectioned and polished under wet condition with increasingly finer grits of SiC papers (#600, #1000, #1200, Carborundum; SaintGobain Abrasivos Ltda, Guarulhos, SP, Brazil). After polishing, bonded interfaces were gently decalcified (37% phosphoric acid for 10 seconds) and deproteinized (2% NaOCl solution for one minute) to evaluate the morphology of the hybrid layer. Specimens were maintained in a desiccator for 48 hours, mounted in aluminum stubs, gold-sputtered (SCD050 Sputter Coater; Balzers, Liechtenstein),
and observed under a scanning electron microscope (1430 VP; LEO Electron Microscopy Ltd., Cambridge, UK) at an accelerating voltage of 20 kV to evaluate the morphology of the bonded interfaces. The exploratory analysis indicated the square root transformation so that data could be analyzed by analysis of variance (ANOVA). The µTBS values were analyzed by 2-way ANOVA (surface pre-treatment vs. resin cement), and multiple comparisons tests were done by Tukey´s HSD test at α=0.05. The statistical program SAS was used.
RESULTS The µTBS results (mean and standard deviation) obtained in all experimental groups are displayed in Table II. Two-way ANOVA (Table III) revealed that only the factor “surface
Table II. Mean µTBS (SD) for each luting agent after different surface treatments Resin Cement Treatment
Maxcem
Multilink Sprint
RelyX Unicem
Control (no pre-treatment)
8.4 (4.5) Ab
6.9 (3.8) Ab
13.8 (8.5) Aa
H3PO4
24.3 (10.8) Aa
21.3 (8.3) Aa
17.4 (8.1) Aa
Clearfil SE Bond
10.5 (5.5) Ab
15.0 (5.0) Aab
16.9 (7.1) Aa
EDTA
6.8 (3.3) Ab
14.8 (5.0) Aab
13.4 (8.2) Aa
Results of post-hoc multiple-comparison tests are indicated by superscripted letters. Within the same column, identical lower case letters indicate significant differences (P<.05). For each row, matching upper case letters indicate significant differences (P<.05). Groups labelled with same superscripted letters are not significantly different (P>.05). Beams that failed prematurely were not included in statistical analysis.
Table III. Two-way analysis of variance for the factors surface pre-treatment and resin cement df
SS
Mean Square
F
P
Treatment
3
87
29
31
<.001
Cement
2
3.3
1.7
1.8
.173
Treatment x Cement
6
30
4.9
5.2
<.001
Error
222
207
0.93
Corrected Total
233
328
Source
P<.05 indicates a significant difference
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Table IV. Distribution (as percentage) of failure modes: A- adhesive, M- mixed, C- cohesive RelyX Unicem
Maxcem
Multilink Sprint
A
M
C
A
M
C
A
M
C
Control (no pre-treatment)
54
37
9
67
32
1
70
30
0
H3PO4
33
51
16
22
60
18
26
58
16
Clearfil SE Bond
36
49
15
62
35
3
46
39
15
EDTA
59
36
5
71
29
0
53
36
11
A
B
C
D
1 SEM micrographs of cement-dentin interfaces luted with RelyX Unicem on no-conditioned, H3PO4, Clearfil SE Bond and EDTA dentin surfaces (RC= resin cement; D= dentin; T= resin tag). A, Superficial interaction of the cement with dentin is observed, as cement irregularities along the interface can be detected (x3000 magnification). B, Luting agent infiltrates H3PO4–etched dentinal tubules to create resin tags (x5000 magnification). C, Application of 2-step self-etch adhesive Clearfil SE Bond resulted in good adaptation of cement to dentin (x3000 magnification). D, Despite formation of resin tags, no sign of hybrid layer formation and cement micro-mechanical inter-locking is detected (x5000 magnification). pre-treatment” (H3PO4 > Clearfil SE Bond > EDTA > no-treatment, P<.001) influenced µTBS values. The interaction between the factors “surface pretreatment” and “resin cement” was also statistically significant (P<.001). H3PO4 -etching achieved the highest bond strength values irrespective of the luting agent. Self-etch
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bonding (with Clearfil SE Bond) and EDTA etching exhibited intermediate bond strengths, while treatment recommended by the manufacturer (no-treatment) recorded the lowest µTBS. Comparing the no-treatment groups only (following manufacturers’ instructions), RelyX Unicem presented statistically higher bond
strengths than Maxcem and Multilink Sprint (P<.001). Bonds obtained for RelyX Unicem were statistically similar for all surface pre-treatments. Generally, failure analysis (Table IV) corroborated the µTBS results, as control (no-pretreatment) specimens failed adhesively. The highest percentages of spontaneous debonding were
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A
B
C
D
2 Representative SEM micrographs of cement-dentin interfaces luted with Maxcem on no-conditioned, H3PO4, Clearfil SE Bond and EDTA dentin surfaces (RC= resin cement; D= dentin; T= resin tag). A, No distinct hybridization of dentin was shown when cement was applied per manufacturers’ recommendations(x5000 magnification). B, When dentin was etched with H3PO4, longer resin tags were observed and cement phase appeared homogeneous (x4000 magnification). C, Thin hybridized layer with few resin tags was visible after Clearfil SE Bond application. Adhesive layer was somewhat detached from cement layer (x5000 magnification). D, Despite formation of resin tags, luting agent layer was completely disconnected from EDTA-etched dentin surface (x5000 magnification).
also observed in these experimental groups. Mixed and cohesive failures were observed in H3PO4-etched groups, which were related to higher bond strengths. SEM observations demonstrated an enhanced potential of the self-adhesive cements to form a hybrid layer when applied following manufacturer’s instructions (Figs. 1A, 2A, and 3A). H3PO4-conditioned dentin interfaces (Figs. 1B, 2B, and 3B) showed well-defined hybrid layers and visible resin tags. Resin tag formation could also be observed in Clearfil SE Bond interfaces (Figs. 1C, 2C, and 3C), with detaching areas between the adhesive and both Maxcem (Fig. 2C) and Multilink Sprint (Fig. 3C)
cement phases. These patterns were not normally found for EDTA preconditioned dentin interfaces (Figs. 1D, 2D, and 3D). Even though resin tag formation could be observed, the cement layer appeared detached from the conditioned dentin surface.
DISCUSSION When bonding with self-adhesive resin cements, the manufacturer’s claim that no dentin pre-treatment is necessary seems to contradict the results of the higher bond strengths that were achieved subsequent to the use of H3PO4-etching. Therefore, the first null hypothesis that dentin surface pre-treatments do not improve
The Journal of Prosthetic Dentistry
bond strengths of self-adhesive resin cements is rejected. In general, polymer bonding is dependent on the surface energy and wettability of the adherent by the adhesive. H3PO4 etching produces a wettable surface due to the removal of the smear layer and an increase in surface roughness that increases wetability.13,14 Therefore, dentin pretreatment with a strong conditioning agent such as phosphoric acid had a beneficial effect on bonding of self-adhesive cements, facilitating the infiltration of the acidic resin monomers into the demineralized dentin substrate. The authors speculate that the increased water content of dentin after phosphoric acid etching10 helps
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A
B
C
D
3 Representative SEM images of cement-dentin interfaces luted with Multilink Sprint on no-conditioned, H3PO4, Clearfil SE Bond and EDTA dentin surfaces (RC= resin cement; D= dentin; T= resin tag). A, No signs of hybrid layer formation were detected and the cement phase exhibited porosities along the interface (x3000 magnification). B, H3PO4-etched interface shows an intimate contact of cement with underlying dentin (x5000 magnification). C, Self-etch bonded interfaces showed clear signs of detachment between adhesive resin and the luting agent (x7000 magnification). D, Poor penetration of cement into EDTA-etched dentin leads to the formation of short resin tags unable to withstand stresses induced by SEM-specimen preparation, causing adhesive failures at the tooth-adhesive interface (x3000 magnification). to better ionize the acidic monomers in these luting agents, permitting an improved surface conditioning and polymer entanglement.15,23,29 SEM interfacial observations of control (Figs. 1A, 2A, and 3A) and H3PO4-treated (Figs. 1B, 2B, and 3B) specimens confirm the bond strength results obtained in the present study. As a consequence, concerns remain regarding the ability of these high viscosity materials to etch through clinically relevant smear layers into the underlying dentin.5 Even though etching tends to increase the wettability of dentin, if the luting agent is too viscous, the cement will not spread over the surface properly. Previous studies reported sig-
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nificantly lower bond strengths for self-adhesive resin cements luted to H3PO4-etched dentin.2,7 Difficulties of standardizing the methodologies such as smear layer thickness and applied load for cement setting, and µTBS specimen preparation, make it impossible to directly compare results presented by the various authors. De Munck et al2 observed a low demineralization effect for RelyX Unicem, even after applying it to fractured dentin surfaces (devoid of smear layer). These results are in agreement with the findings of the present study. The bond strength of resin cements is lower than that of adhesive systems for direct composite resins.20 In light of these findings, the 2-step,
self-etching adhesive Clearfil SE Bond was elected as dentin pretreatment, because of its improved bond strength performance19 and low technique sensitivity.13 In the present study, however, this adhesive did not promote a significant µTBS increase when compared to the manufacturer’s mode of application (no treatment). It must be stressed that SEM examinations of Clearfil SE Bond-treated interfaces revealed that dentin tubules were sealed by resin tags, but only showed an intimate entanglement with RelyX Unicem (Fig. 1C). However, interfaces luted with Maxcem (Fig. 2C) and Multilink Sprint (Fig. 3C) remained sealed by the adhesive, but presented voids at the adhesive-cement in-
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Volume 105 Issue 4 terface, thus corroborating the low bond strength values attained. Even when ternary redox catalysts are present in the adhesives and/or resin cements,11 increases in adhesive permeability may result in adhesive–cement incompatibility, and are likely to be the primary reason for this premature decoupling. To overcome this problem, a ‘resin coating’ technique could be recommended.30 EDTA promotes a milder demineralization of dentin, but for the selfadhesive resin cements, it was not sufficiently able to create spaces for polymer infiltration and micromechanical retention (Figs. 1D, 2D, and 3D). Furthermore, EDTA is a molecule containing 4 carboxylic acid groups that has the ability to chelate calcium.24 In medicine, EDTA is used in chelation therapy for acute hypercalcemia.31 Therefore, Ca+ ion depletion onto dentin surface may be expected due to EDTA’s scavenging property. As relatively less Ca+ ions are available on the tooth surface after EDTA application, the potential chemical bonding that is expected with self-adhesive luting agents decrease and impair the final bond strength. Even though some have advocated the use of EDTA to substitute for H3PO4 adhesive systems,24,32 care must be taken in evaluation of self-adhesive cement adhesive joints. Bond strength results (Table III), fracture pattern (Table II) and interfacial morphologic evaluations suggest that a stronger conditioning protocol is necessary to promote both cement infiltration and chemical bonding to tooth substrates. It is likely that EDTA did not penetrate the smear layer in 60 seconds of treatment. The results do not support rejection of the second null hypothesis that higher µTBS does not depend on the luting agent used, as µTBS values were not material-dependent. RelyX Unicem was the least influenced by the different dentin pre-treatments. The formulations and properties of self-adhesive resin cements are similar to composite resins.21 All tested ce-
ments contained a phosphoric acid ester monomer (Table I). However, variations in chemical and mechanical properties of the bonded cement, such as formulation, tensile strength, flexural strength and Young’s modulus5, 27 are important for mechanical characteristics such as microtensile bond strength. RelyX Unicem contains glass particles in the ingredients, which indicates that some characteristics of glass-ionomer cement could be expected.6 Gerth et al5 observed an increased chemical interaction with Ca+ from Hap and improved polymerization for RelyX Unicem in comparison to other self-adhesive resin cement. This feature could help to explain the more stable bonding properties of RelyX Unicem compared to the tested self-adhesive luting agents in the present study. Nevertheless, no detectable hybrid layer was observed when RelyX Unicem was luted to untreated dentin. This cement presented a superficial, but continuous interaction with the underlying dentin (Fig. 1A), differently from MaxCem (Fig. 2A) and Multilink (Fig. 3A) interfaces, that were completely debonded from the superficial dentin. The chemical compositions of resin cements may differentiate their bonding behavior when applied after different dentin surface pre-treatments, significantly affecting their bond strength performance. Even so, regardless the composition, selfadhesive resin cements only interact with dentin superficially without any appearance of a hybrid layer or resin tags, demonstrating a poor interaction with the underlying dentin.2,25,26 SEM observations of untreated dentin surfaces (Figs. 1A, 2A, and 3A) confirm that the bonding mechanism of these simplified cements to dentin needs further improvements (such as improved infiltration and chemical binding to dentin), as no distinct demineralization and true hybridization could be observed, when compared to H3PO4- (Figs. 1B, 2B, and 3B) and self-etch (Figs. 1D, 2D, and 3D) treated groups. The presence of resin tags
The Journal of Prosthetic Dentistry
in dentinal tubules means that the tubule orifices were patent enough for resin to flow into the tubule and to hybridize with surrounding collagen fibrils, allowing better sealing. This more intense interaction seems to be relevant to clinical aspects. However, it is unclear whether the mechanical properties of composite resins are the result of a chemical interaction with the teeth components and/or of micro-mechanical inter-locking type. 5 Although self-adhesive cements are thought to etch and prime the dentin surface simultaneously, the bond strength results attained in the present study confirm that previous H3PO4 etching positively influences self-adhesive cements bond strength, based on a two-fold mechanism: (1) increased micro-mechanical interlocking that helps the infiltration of the resin monomers through the demineralized interfibrillar spaces, and (2) the promotion of a dentinal water outflow that enhances the cement acid-base reaction and as a consequence its chemical interaction with the dentin surface. Simplification may ease handling for the clinician, but may not improve bonding effectiveness of self-adhesive resin cements. It remains to be determined whether dentin treatment with strong acids prior to luting with simplified cements results in long-term higher bond strengths, as well as the creation of hybrid layers of improved quality to dentin substrates. It must be stressed that in vitro testing is more simplified than the in vivo situation. Teeth in the oral environment are continually subjected to different types of stresses that may impair the bonding effectiveness of the luting agents.2 Factors such as pH changes, occlusal loading and enzymatic challenges may also contribute to the physico-chemical degradation of resin-dentin bonds. The attained results highlight the need for investigations on the long-lasting performance of simplified adhesives with more stable characteristics after polymerization and ability to promote a
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April 2011 durable bond to a complex substrate such as caries-affected dentin. Thus, further long-term investigations and clinical trials are desirable to improve adhesive dentistry, not only by developing less time-consuming materials, but also by enhancing the quality and longevity of resin bonds created in dental treatments.
CONCLUSIONS Within the limitations of this study, it is concluded that dentin pre-treatment with phosphoric acid has a positive influence when luting with self-adhesive resin-based cements. Therefore, omitting dentin pre-treatment procedures, as recommended by manufacturers, may not be the most appropriate technique for dentin luting.
REFERENCES 1. Radovic I, Monticelli F, Goracci C, Vulicevic R, Ferrari M. Self-adhesive resin cements: A literature review. J Adhes Dent 2008;10:251-8. 2. De Munck J, Vargas M, Van Landuyt K, Hikita K, Lambrechts P, Van Meerbeek B. Bonding of an auto-adhesive luting material to enamel and dentin. Dent Mater 2004;20:963-71. 3. Gladys S, Van Meerbeek B, Braem M, Lambrechts P, Vanherle G. Comparative physico-mechanical characterization of new hybrid restorative materials with conventional glass-ionomer and resin composite restorative materials. J Dent Res 1997;76:883-94. 4. Abo-Hamar SE, Hiller KA, Jung H, Federlin M, Friedl KH, Schmalz G. Bond strength of a new universal self-adhesive resin luting cement to dentin and enamel. Clin Oral Investig 2005;9:161-7. 5. Gerth HU, Dammaschke T, Züchner H, Schäfer E. Chemical analysis and bonding reaction of RelyX Unicem and Bifix composites - a comparative study. Dent Mater 2006;22:934-41. 6. Han L, Okamoto A, Fukushima M, Okiji T. Evaluation of physical properties and surface degradation of self-adhesive resin cements. Dent Mater J 2007;26:906-14. 7. Hikita K, Van Meerbeek B, De Munck J, Ikeda T, Van Landuyt K, Maida T, et al. Bonding effectiveness of adhesive luting agents to enamel and dentin. Dent Mater 2007;23:71-80.
8. Yang B, Ludwig K, Adelung R, Kern M. Micro-tensile bond strength of three luting resins to human regional dentin. Dent Mater 2006;22:45-56. 9. Marshall GW Jr, Marshall SJ, Kinney JH, Balooch M. The dentin substrate: structure and properties related to bonding. J Dent 1997;25:441-58. 10.Banomyong D, Palamara JE, Burrow MF, Messer HH. Effect of dentin conditioning on dentin permeability and micro-shear bond strength. Eur J Oral Sci 2007;115:502-9. 11.Carvalho RM, Pegoraro TA, Tay FR, Pegoraro LF, Silva NR, Pashley DH. Adhesive permeability affects coupling of resin cements that utilise self-etching primers to dentine. J Dent 2004;32:55-65. 12.Carvalho RM, Chersoni S, Frankenberger R, Pashley DH, Prati C, Tay FR. A challenge to the conventional wisdom that simultaneous etching and resin infiltration always occurs in self-etch adhesives. Biomaterials 2005;26:1035-42. 13.Van Meerbeek B, De Munck J, Yoshida Y, Inoue S, Vargas M, Vijay P, et al. Buonocore memorial lecture. Adhesion to enamel and dentin: current status and future challenges. Oper Dent 2003;28:215–35. 14.Toledano M, Osorio R, Perdigao J, Rosales JI, Thompson JY, Cabrerizo-Vilchez MA. Effect of acid etching and collagen removal on dentin wettability and roughness. J Biomed Mater Res 1999;47:198-203. 15.Erhardt MC, Cavalcante LM, Pimenta LA. Influence of phosphoric acid pretreatment on self-etching bond strengths. J Esthet Restor Dent 2004;16:33-40. 16.Pashley DH, Carvalho RM. Dentine permeability and dentine adhesion. J Dent 1997;25:355-72. 17.Tay FR, Pashley DH. Aggressiveness of contemporary self-etching systems. I: Depth of penetration beyond dentin smear layers. Dent Mater 2001;17:296-308. 18.Nakabayashi N, Saimi Y. Bonding to intact dentin. J Dent Res 1996;75:1706-15. 19.Proença JP, Polido M, Osorio E, Erhardt MC, Aguilera FS, García-Godoy F, et al. Dentin regional bond strength of self-etch and total-etch adhesive systems. Dent Mater 2007;23:1542-8. 20.Jayasooriya PR, Pereira PN, Nikaido T, Tagami J. Efficacy of a resin coating on bond strengths of resin cement to dentin. J Esthet Restor Dent 2003;15:105-13. 21.Kamada K, Taira Y, Yoshida K, Atsuta M. Effect of four silane coupling agents on bonding of two resin-modified glass ionomer cement to a machinable ceramic. Dent Mater J 2007;26:240-4. 22.Kern M, Thompson VP. Bonding to glass infiltrated alumina ceramic: adhesive methods and their durability. J Prosthet Dent 1995;73:240-9.
23.Mazzitelli C, Monticelli F, Osorio R, Casucci A, Toledano M, Ferrari M. Effect of simulated pulpal pressure on self-adhesive cements bonding to dentin. Dent Mater 2008;24:1156-63. 24.Osorio R, Erhardt MC, Pimenta LA, Osorio E, Toledano M. EDTA treatment of dentin improves resin-dentin bonds durability. J Dent Res 2005;84:736-40. 25.Al-Assaf K, Chakmakchi M, Palaghias G, Karanika-Kouma A, Eliades G. Interfacial characteristics of adhesive luting resins and composites with dentine. Dent Mater 2007;23:829-39. 26.Goracci C, Cury AH, Cantoro A, Papacchini F, Tay FR, Ferrari M. Microtensile bond strength and interfacial properties of self-etching and self-adhesive resin cements used to lute composite onlays under different seating forces. J Adhes Dent 2006;8:327-35 27.Hasegawa T, Itoh K, Koike T, Yukitani W, Hisamitsu H, Wakumoto S, et al. Effect of mechanical properties of resin composites on the efficacy of the dentin bonding system. Oper Dent 1999;24:323-30. 28.Gogos C, Stavrianos C, Kolokouris I, Economides N, Papadoyannis I. Shear bond strength of two resin cements to human root dentin using three dentin bonding agents. Oper Dent 2007;32:31-6. 29.Moszner N, Salz U, Zimmermann J. Chemical aspects of self-etching enamel-dentin adhesives: a systematic review. Dent Mater 2005;21:895-910. 30.Sultana S, Nikaido T, Matin K, Ogata M, Foxton RM, Tagami J. Effect of resin coating on dentin bonding of resin cement in Class II cavities. Dent Mater J 2007;26:506-13. 31.Kwon YS, Song YS, Kim JC. New treatment for treatment keratopathy: superficial lamellar keratectomy, EDTA chelation and amniotic membrane transplantation. J Korean Med Sci 2004;19:611-5. 32.Torii Y, Hikasa R, Iwate S, Oyama F, Itou K, Yoshiyama M. Effect of EDTA conditioning on bond strength to bovine dentin promoted by four current adhesives. Am J Dent 2003;16:395-400. Corresponding author: Dr Maria Carolina Guilherme Erhardt Federal University of Rio Grande do Sul – School of Dentistry Department of Conservative Dentistry Rua Ramiro Barcelos, 2492 90035-003, Porto Alegre – RS BRAZIL Fax: +55-51-33085202 E-mail: [email protected] Acknowledgments The authors thank Professor Glaucia Maria Bovi Ambrosano (State University of Campinas, UNICAMP, Brazil) for support with the statistical analysis. Copyright © 2011 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Research Fellow, Department of Stomatology, School of Dentistry, University of Granada. Associate Professor, Department of Conservative Dentistry, School of Dentistry, Federal University of Rio Grande do Sul. c Research Fellow, Department of Dental Materials and Prosthodontics, São Paulo State University at São Jose dos Campos. d Associate Professor, Department of Restorative Dentistry, School of Dentistry, Federal University of Santa Maria. e Chair and Professor, Department of Dental Materials and Prosthodontics, São Paulo State University at São Jose dos Campos. f Professor, Department of Prosthodontics, School of Dentistry, University of Granada. a
b
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Clinical Implications
Bond strength of evaluated self-adhesive resin cements is improved by pre-conditioning the dentin surface with H3PO4. Sufficient smear layer dissolution is necessary to enable chemical interaction and micro-mechanical inter-locking with the dentin surface. The presumed benefit of saving time with self-adhesive luting agents may only be realized at the expense of compromising bond strength. There is a growing interest in the use of self-adhesive resin cements. Manufacturers purport to combine an adhesive and a cement in a single application, eliminating the need for pre-treatment of both tooth and restoration.1 Bonding is thought to be primarily achieved by acidic monomers that demineralize and infiltrate the tooth substrate, resulting in mechanical retention.2 Secondary reactions have been suggested to provide chemical adhesion to hydroxyapatite, a feature currently only demonstrated in glass-ionomers.3 The simplification in handling achieved with these new agents is expected to make the luting procedure less technique- and operator-sensitive than when using 3-step systems.2,4 Thus, the concept of self-adhesive luting is attractive, because in theory, it is assumed that these cements should adhesively bond to smear layer-covered dentin without any pretreatment.1,2 Nevertheless, a limited etching potential and the ability to only superficially interact with dentin were observed for some self-adhesive resin cements,5-7 related to the high viscosity of the cement that would hinder deeper resin penetration.8 The diffusion of acidic resin monomers through the smear layer is relatively slow.9 Smear layers reinforced by impregnated resin may be too weak to provide strong, durable bonds.10-13 The mineralized components of the smear layer are efficient buffers,9,14 making the pH of the acidic monomers too high to demineralize the underlying dentin.15-17 Undeniably, more stable bonds may be formed if polymers can penetrate through den-
tal substrates less deeply, but more uniformly.18,19 The bonding mechanisms of different composite resins and adhesive monomers to tooth structures are the topic of several investigations.20-27 Therefore, it would be worthwhile to investigate if alternative bonding strategies,28-30 such as the use of different smear layer conditioning solutions (for example, strong or milder acids, self-etching adhesives) may be helpful in achieving better infiltration of dentin by self-adhesive cements, and as a consequence, improve the bond strength. The purpose of this study was to assess the impact of different conditioning agents, phosphoric acid, a mild self-etching adhesive system and EDTA, on the microtensile bond strength (µTBS) of 3 self-adhesive resin-based luting agents to dentin. The interaction of such bonding materials with dentin was also examined by scanning electron microscopy (SEM). The null hypotheses to be tested were: (1) dentin surface pre-treatments do not improve bond strengths of selfadhesive resin cements, and (2) higher µTBS does not depend on the luting agent used.
MATERIAL AND METHODS Forty-eight caries-free extracted human molars were used in this study. Teeth were stored in distilled water containing 0.5% chloramine T (Haldies Chemicals Pvt Ltd, Pune, India) at 4°C, and used within one month following extraction. The occlusal surfaces of the molars were ground flat with silicon carbide (SiC) metal-
The Journal of Prosthetic Dentistry
lographic papers (600-grit, Carborundum; Saint-Gobain Abrasivos Ltda, Guarulhos, SP, Brazil) under running water to obtain smear layer-covered dentin surfaces. Prior to the cementing procedures, a wax block (Cera Reus, S.A., Reus, Spain), 6 x 5 x 5 mm, was fabricated and subsequently impressed with vinyl polysiloxane (VPS) impression material (Elite HD; Zhermack, Badia Polesine, Italy), resulting in a matrix of VPS. A light-polymerized hybrid composite resin (Tetric EvoCeram, lot no. F38346; Ivoclar Vivadent, Schaan, Liechtenstein,) was incrementally inserted (2 mm), condensed into the templates and light polymerized for 40 seconds (Translux EC; Heraeus Kulzer GmbH, Hanau, Germany) until each template was completely filled. Light intensity output was monitored with a radiometer (Demetron Curing Radiometer; Kerr Corp, Orange, Calif ) throughout the bonding procedures to maintain at least 600 mW/ cm2. Polymerized composite resin blocks were polished with SiC papers up to 1200-grit under copious water and further cleaned ultrasonically (Vitasonic II; Vita Zanhfabrik, Bad Säckingen, Germany) for 5 minutes. The 48 flattened dentin surfaces were divided into 4 groups, according to the dentin pre-treatment to be applied (Table I): 1) no surface conditioning (control), the resin cements were applied directly to moist smear-layer covered dentin surfaces; 2) H3PO4, prior to luting, dentin was treated with 37% phosphoric acid (Total Etch; Ivoclar Vivadent) for 15 seconds, water rinsed for 10 seconds and left moist; 3) SEBond, a 2-step
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Table I. Manufacturers and composition of materials used in study Material (pH) RelyX
Manfacturer (Lot Number)
Composition
Application
3M ESPE,
Powder: glass powder, silica, calcium hydroxide,
Mix capsule for 15 s; apply on surface;
pigment, substituted pyrimidine, peroxy
lute composite resin block using pressure;
compound, initiator.
light polymerize for 40 s from each side.
Unicem
St. Paul, Minn
(pH 2.0)
(249122)
Liquid: methacrylated phosphoric ester, dimethacrylate, acetate, stabilizer, initiator. Maxcem
Kerr Corp,
(pH 2.3)
Orange, Calif (443517)
Unpolymerized methacrylate ester monomers,
Mix base and catalyst through automix
mineral fillers, activators, stabilizers and
dual-barrel syringe; apply on surface;
colorants.
lute composite resin block using pressure; light polymerize for 40 s from each side.
Multilink Sprint (pH 2.8)
Ivoclar Vivadent, Schaan, Liechtenstein (experimental lot)
Base and Catalyst: pastes of dimethacrylates
Auto-mix cement; apply on surface;
(24-26%), inorganic fillers, ytterbiumtrifluoride,
lute composite resin block using pressure;
initiators, stabilizers and pigments
light polymerize for 40 s from each side.
(5% methacrylated phosphoric acid ester). Total Etch
Ivoclar Vivadent,
(pH 0.6)
Schaan, Liechtenstein
35% phosphoric acid, fumared silica.
H3PO4 conditioning for 15 s. Rinse with water spray for 10 s, leaving tooth moist.
(JO8736) Clearfil SE
Kuraray Co,
Primer: 10-MDP; HEMA; hydrophilic
Apply Primer for 20 s; mild air stream.
Bond
Osaka, Japan
dimethacrylate; di-camphorquinone;
Apply Adhesive; gentle air stream.
N,N-diethanol-p-toludine, water.
Light polymerize for 10 s.
(00604A)
Adhesive: 10-MDP; Bis-GMA; HEMA; hydrophobic dimethacrylate; di-camphorquinone; N,N-diethanol-p-toluidine; silanated colloidal silica. 10-MDP=10-methacryloxydecyl dihydrogen phosphate; HEMA=2-hydroxyethyl methacrylate; Bis-GMA=bis-phenol A diglycidylmethacrylate.
self-etching primer adhesive (Clearfil SE Bond; Kuraray Co, LTD, Osaka, Japan) was bonded to the dentin prior to the luting procedures according to the manufacturer’s instructions; 4) EDTA, a 0.1 M EDTA (ethylenediaminetetraacetic acid) solution (pH 7) was placed onto dentin surfaces for 60 seconds; the surface was then rinsed with water spray for 10 seconds and left moist. After dentin surface treatments, teeth were re-assigned into 3 subgroups (n=4). Three self-adhesive luting agents were used: 1) RelyX Uni-
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cem, 2) Maxcem, and 3) Multilink Sprint. Materials were manipulated according to the manufacturer´s instructions. Composition, pH values and application protocol of the tested materials are listed in Table I. The pre-polymerized composite resin blocks were directly pressed on the cement using strong finger pressure and stabilized under a 750-g static load applied for 10 minutes.22 The excess cement was removed with a brush before light polymerizing with a halogen unit (Translux EC; Heraeus Kulzer GmbH) for four 40-second periods at
each side of the cemented composite resin blocks. Light intensity output was monitored with a radiometer (Demetron Curing Radiometer, Model 100; Kerr Corp) throughout the bonding procedures to be at least 600 mW/ cm2, and the light tip was positioned at 0.5 cm from the specimens during the polymerization process. After storage in distilled water at 37°C for 7 days, the resin-cementdentin specimens were longitudinally sectioned into serial slabs, and further sectioned into 1.0 ± 0.2 mm2 beams. Beams from each experimental group
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Volume 105 Issue 4 were individually attached to a microtensile bond strength apparatus with a cyanoacrylate adhesive (Super Bonder Gel; Henkel Loctite, Düsseldorf, Germany) and stressed to failure in tension using a universal testing machine (Model 5567; Instron Corp, Norwood, Mass) with a 10 Kgf load cell operated at a crosshead speed of 1.0 mm/min. The cross-sectional area at the site of failure was measured to the nearest 0.01 mm2 with a digital caliper (Starrett 727; L.S. Starrett, Athol, Mass). Fractured specimens were examined with a stereomicroscope (Olympus SZ-CTV; Olympus, Tokyo, Japan) at x40 magnification to determine the mode of failure. Failure modes were classified as adhesive, mixed, or cohesive. In the event of spontaneous debonding during handling, specimens were excluded from the statistical analysis.
Two additional resin-cementdentin specimens from each experimental group were prepared as previously described, and subsequently processed for morphological evaluation by scanning electron microscopy. Two mm-thick slabs containing the coupled interfaces were sectioned and polished under wet condition with increasingly finer grits of SiC papers (#600, #1000, #1200, Carborundum; SaintGobain Abrasivos Ltda, Guarulhos, SP, Brazil). After polishing, bonded interfaces were gently decalcified (37% phosphoric acid for 10 seconds) and deproteinized (2% NaOCl solution for one minute) to evaluate the morphology of the hybrid layer. Specimens were maintained in a desiccator for 48 hours, mounted in aluminum stubs, gold-sputtered (SCD050 Sputter Coater; Balzers, Liechtenstein),
and observed under a scanning electron microscope (1430 VP; LEO Electron Microscopy Ltd., Cambridge, UK) at an accelerating voltage of 20 kV to evaluate the morphology of the bonded interfaces. The exploratory analysis indicated the square root transformation so that data could be analyzed by analysis of variance (ANOVA). The µTBS values were analyzed by 2-way ANOVA (surface pre-treatment vs. resin cement), and multiple comparisons tests were done by Tukey´s HSD test at α=0.05. The statistical program SAS was used.
RESULTS The µTBS results (mean and standard deviation) obtained in all experimental groups are displayed in Table II. Two-way ANOVA (Table III) revealed that only the factor “surface
Table II. Mean µTBS (SD) for each luting agent after different surface treatments Resin Cement Treatment
Maxcem
Multilink Sprint
RelyX Unicem
Control (no pre-treatment)
8.4 (4.5) Ab
6.9 (3.8) Ab
13.8 (8.5) Aa
H3PO4
24.3 (10.8) Aa
21.3 (8.3) Aa
17.4 (8.1) Aa
Clearfil SE Bond
10.5 (5.5) Ab
15.0 (5.0) Aab
16.9 (7.1) Aa
EDTA
6.8 (3.3) Ab
14.8 (5.0) Aab
13.4 (8.2) Aa
Results of post-hoc multiple-comparison tests are indicated by superscripted letters. Within the same column, identical lower case letters indicate significant differences (P<.05). For each row, matching upper case letters indicate significant differences (P<.05). Groups labelled with same superscripted letters are not significantly different (P>.05). Beams that failed prematurely were not included in statistical analysis.
Table III. Two-way analysis of variance for the factors surface pre-treatment and resin cement df
SS
Mean Square
F
P
Treatment
3
87
29
31
<.001
Cement
2
3.3
1.7
1.8
.173
Treatment x Cement
6
30
4.9
5.2
<.001
Error
222
207
0.93
Corrected Total
233
328
Source
P<.05 indicates a significant difference
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Table IV. Distribution (as percentage) of failure modes: A- adhesive, M- mixed, C- cohesive RelyX Unicem
Maxcem
Multilink Sprint
A
M
C
A
M
C
A
M
C
Control (no pre-treatment)
54
37
9
67
32
1
70
30
0
H3PO4
33
51
16
22
60
18
26
58
16
Clearfil SE Bond
36
49
15
62
35
3
46
39
15
EDTA
59
36
5
71
29
0
53
36
11
A
B
C
D
1 SEM micrographs of cement-dentin interfaces luted with RelyX Unicem on no-conditioned, H3PO4, Clearfil SE Bond and EDTA dentin surfaces (RC= resin cement; D= dentin; T= resin tag). A, Superficial interaction of the cement with dentin is observed, as cement irregularities along the interface can be detected (x3000 magnification). B, Luting agent infiltrates H3PO4–etched dentinal tubules to create resin tags (x5000 magnification). C, Application of 2-step self-etch adhesive Clearfil SE Bond resulted in good adaptation of cement to dentin (x3000 magnification). D, Despite formation of resin tags, no sign of hybrid layer formation and cement micro-mechanical inter-locking is detected (x5000 magnification). pre-treatment” (H3PO4 > Clearfil SE Bond > EDTA > no-treatment, P<.001) influenced µTBS values. The interaction between the factors “surface pretreatment” and “resin cement” was also statistically significant (P<.001). H3PO4 -etching achieved the highest bond strength values irrespective of the luting agent. Self-etch
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bonding (with Clearfil SE Bond) and EDTA etching exhibited intermediate bond strengths, while treatment recommended by the manufacturer (no-treatment) recorded the lowest µTBS. Comparing the no-treatment groups only (following manufacturers’ instructions), RelyX Unicem presented statistically higher bond
strengths than Maxcem and Multilink Sprint (P<.001). Bonds obtained for RelyX Unicem were statistically similar for all surface pre-treatments. Generally, failure analysis (Table IV) corroborated the µTBS results, as control (no-pretreatment) specimens failed adhesively. The highest percentages of spontaneous debonding were
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A
B
C
D
2 Representative SEM micrographs of cement-dentin interfaces luted with Maxcem on no-conditioned, H3PO4, Clearfil SE Bond and EDTA dentin surfaces (RC= resin cement; D= dentin; T= resin tag). A, No distinct hybridization of dentin was shown when cement was applied per manufacturers’ recommendations(x5000 magnification). B, When dentin was etched with H3PO4, longer resin tags were observed and cement phase appeared homogeneous (x4000 magnification). C, Thin hybridized layer with few resin tags was visible after Clearfil SE Bond application. Adhesive layer was somewhat detached from cement layer (x5000 magnification). D, Despite formation of resin tags, luting agent layer was completely disconnected from EDTA-etched dentin surface (x5000 magnification).
also observed in these experimental groups. Mixed and cohesive failures were observed in H3PO4-etched groups, which were related to higher bond strengths. SEM observations demonstrated an enhanced potential of the self-adhesive cements to form a hybrid layer when applied following manufacturer’s instructions (Figs. 1A, 2A, and 3A). H3PO4-conditioned dentin interfaces (Figs. 1B, 2B, and 3B) showed well-defined hybrid layers and visible resin tags. Resin tag formation could also be observed in Clearfil SE Bond interfaces (Figs. 1C, 2C, and 3C), with detaching areas between the adhesive and both Maxcem (Fig. 2C) and Multilink Sprint (Fig. 3C)
cement phases. These patterns were not normally found for EDTA preconditioned dentin interfaces (Figs. 1D, 2D, and 3D). Even though resin tag formation could be observed, the cement layer appeared detached from the conditioned dentin surface.
DISCUSSION When bonding with self-adhesive resin cements, the manufacturer’s claim that no dentin pre-treatment is necessary seems to contradict the results of the higher bond strengths that were achieved subsequent to the use of H3PO4-etching. Therefore, the first null hypothesis that dentin surface pre-treatments do not improve
The Journal of Prosthetic Dentistry
bond strengths of self-adhesive resin cements is rejected. In general, polymer bonding is dependent on the surface energy and wettability of the adherent by the adhesive. H3PO4 etching produces a wettable surface due to the removal of the smear layer and an increase in surface roughness that increases wetability.13,14 Therefore, dentin pretreatment with a strong conditioning agent such as phosphoric acid had a beneficial effect on bonding of self-adhesive cements, facilitating the infiltration of the acidic resin monomers into the demineralized dentin substrate. The authors speculate that the increased water content of dentin after phosphoric acid etching10 helps
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April 2011
A
B
C
D
3 Representative SEM images of cement-dentin interfaces luted with Multilink Sprint on no-conditioned, H3PO4, Clearfil SE Bond and EDTA dentin surfaces (RC= resin cement; D= dentin; T= resin tag). A, No signs of hybrid layer formation were detected and the cement phase exhibited porosities along the interface (x3000 magnification). B, H3PO4-etched interface shows an intimate contact of cement with underlying dentin (x5000 magnification). C, Self-etch bonded interfaces showed clear signs of detachment between adhesive resin and the luting agent (x7000 magnification). D, Poor penetration of cement into EDTA-etched dentin leads to the formation of short resin tags unable to withstand stresses induced by SEM-specimen preparation, causing adhesive failures at the tooth-adhesive interface (x3000 magnification). to better ionize the acidic monomers in these luting agents, permitting an improved surface conditioning and polymer entanglement.15,23,29 SEM interfacial observations of control (Figs. 1A, 2A, and 3A) and H3PO4-treated (Figs. 1B, 2B, and 3B) specimens confirm the bond strength results obtained in the present study. As a consequence, concerns remain regarding the ability of these high viscosity materials to etch through clinically relevant smear layers into the underlying dentin.5 Even though etching tends to increase the wettability of dentin, if the luting agent is too viscous, the cement will not spread over the surface properly. Previous studies reported sig-
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nificantly lower bond strengths for self-adhesive resin cements luted to H3PO4-etched dentin.2,7 Difficulties of standardizing the methodologies such as smear layer thickness and applied load for cement setting, and µTBS specimen preparation, make it impossible to directly compare results presented by the various authors. De Munck et al2 observed a low demineralization effect for RelyX Unicem, even after applying it to fractured dentin surfaces (devoid of smear layer). These results are in agreement with the findings of the present study. The bond strength of resin cements is lower than that of adhesive systems for direct composite resins.20 In light of these findings, the 2-step,
self-etching adhesive Clearfil SE Bond was elected as dentin pretreatment, because of its improved bond strength performance19 and low technique sensitivity.13 In the present study, however, this adhesive did not promote a significant µTBS increase when compared to the manufacturer’s mode of application (no treatment). It must be stressed that SEM examinations of Clearfil SE Bond-treated interfaces revealed that dentin tubules were sealed by resin tags, but only showed an intimate entanglement with RelyX Unicem (Fig. 1C). However, interfaces luted with Maxcem (Fig. 2C) and Multilink Sprint (Fig. 3C) remained sealed by the adhesive, but presented voids at the adhesive-cement in-
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Volume 105 Issue 4 terface, thus corroborating the low bond strength values attained. Even when ternary redox catalysts are present in the adhesives and/or resin cements,11 increases in adhesive permeability may result in adhesive–cement incompatibility, and are likely to be the primary reason for this premature decoupling. To overcome this problem, a ‘resin coating’ technique could be recommended.30 EDTA promotes a milder demineralization of dentin, but for the selfadhesive resin cements, it was not sufficiently able to create spaces for polymer infiltration and micromechanical retention (Figs. 1D, 2D, and 3D). Furthermore, EDTA is a molecule containing 4 carboxylic acid groups that has the ability to chelate calcium.24 In medicine, EDTA is used in chelation therapy for acute hypercalcemia.31 Therefore, Ca+ ion depletion onto dentin surface may be expected due to EDTA’s scavenging property. As relatively less Ca+ ions are available on the tooth surface after EDTA application, the potential chemical bonding that is expected with self-adhesive luting agents decrease and impair the final bond strength. Even though some have advocated the use of EDTA to substitute for H3PO4 adhesive systems,24,32 care must be taken in evaluation of self-adhesive cement adhesive joints. Bond strength results (Table III), fracture pattern (Table II) and interfacial morphologic evaluations suggest that a stronger conditioning protocol is necessary to promote both cement infiltration and chemical bonding to tooth substrates. It is likely that EDTA did not penetrate the smear layer in 60 seconds of treatment. The results do not support rejection of the second null hypothesis that higher µTBS does not depend on the luting agent used, as µTBS values were not material-dependent. RelyX Unicem was the least influenced by the different dentin pre-treatments. The formulations and properties of self-adhesive resin cements are similar to composite resins.21 All tested ce-
ments contained a phosphoric acid ester monomer (Table I). However, variations in chemical and mechanical properties of the bonded cement, such as formulation, tensile strength, flexural strength and Young’s modulus5, 27 are important for mechanical characteristics such as microtensile bond strength. RelyX Unicem contains glass particles in the ingredients, which indicates that some characteristics of glass-ionomer cement could be expected.6 Gerth et al5 observed an increased chemical interaction with Ca+ from Hap and improved polymerization for RelyX Unicem in comparison to other self-adhesive resin cement. This feature could help to explain the more stable bonding properties of RelyX Unicem compared to the tested self-adhesive luting agents in the present study. Nevertheless, no detectable hybrid layer was observed when RelyX Unicem was luted to untreated dentin. This cement presented a superficial, but continuous interaction with the underlying dentin (Fig. 1A), differently from MaxCem (Fig. 2A) and Multilink (Fig. 3A) interfaces, that were completely debonded from the superficial dentin. The chemical compositions of resin cements may differentiate their bonding behavior when applied after different dentin surface pre-treatments, significantly affecting their bond strength performance. Even so, regardless the composition, selfadhesive resin cements only interact with dentin superficially without any appearance of a hybrid layer or resin tags, demonstrating a poor interaction with the underlying dentin.2,25,26 SEM observations of untreated dentin surfaces (Figs. 1A, 2A, and 3A) confirm that the bonding mechanism of these simplified cements to dentin needs further improvements (such as improved infiltration and chemical binding to dentin), as no distinct demineralization and true hybridization could be observed, when compared to H3PO4- (Figs. 1B, 2B, and 3B) and self-etch (Figs. 1D, 2D, and 3D) treated groups. The presence of resin tags
The Journal of Prosthetic Dentistry
in dentinal tubules means that the tubule orifices were patent enough for resin to flow into the tubule and to hybridize with surrounding collagen fibrils, allowing better sealing. This more intense interaction seems to be relevant to clinical aspects. However, it is unclear whether the mechanical properties of composite resins are the result of a chemical interaction with the teeth components and/or of micro-mechanical inter-locking type. 5 Although self-adhesive cements are thought to etch and prime the dentin surface simultaneously, the bond strength results attained in the present study confirm that previous H3PO4 etching positively influences self-adhesive cements bond strength, based on a two-fold mechanism: (1) increased micro-mechanical interlocking that helps the infiltration of the resin monomers through the demineralized interfibrillar spaces, and (2) the promotion of a dentinal water outflow that enhances the cement acid-base reaction and as a consequence its chemical interaction with the dentin surface. Simplification may ease handling for the clinician, but may not improve bonding effectiveness of self-adhesive resin cements. It remains to be determined whether dentin treatment with strong acids prior to luting with simplified cements results in long-term higher bond strengths, as well as the creation of hybrid layers of improved quality to dentin substrates. It must be stressed that in vitro testing is more simplified than the in vivo situation. Teeth in the oral environment are continually subjected to different types of stresses that may impair the bonding effectiveness of the luting agents.2 Factors such as pH changes, occlusal loading and enzymatic challenges may also contribute to the physico-chemical degradation of resin-dentin bonds. The attained results highlight the need for investigations on the long-lasting performance of simplified adhesives with more stable characteristics after polymerization and ability to promote a
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April 2011 durable bond to a complex substrate such as caries-affected dentin. Thus, further long-term investigations and clinical trials are desirable to improve adhesive dentistry, not only by developing less time-consuming materials, but also by enhancing the quality and longevity of resin bonds created in dental treatments.
CONCLUSIONS Within the limitations of this study, it is concluded that dentin pre-treatment with phosphoric acid has a positive influence when luting with self-adhesive resin-based cements. Therefore, omitting dentin pre-treatment procedures, as recommended by manufacturers, may not be the most appropriate technique for dentin luting.
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